Method for making semiconductor device including a superlattice and enriched silicon 28 epitaxial layer

ABSTRACT

A method for making a semiconductor device may include forming a first single crystal silicon layer having a first percentage of silicon 28, and forming a superlattice above the first single crystal silicon layer. The superlattice may include a plurality of stacked groups of layers, with each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base silicon portions. The method may further include forming a second single crystal silicon layer above the superlattice having a second percentage of silicon 28 higher than the first percentage of silicon 28.

TECHNICAL FIELD

The present disclosure generally relates to semiconductor devices and, more particularly, to semiconductor devices with enhanced semiconductor materials and associated methods.

BACKGROUND

Structures and techniques have been proposed to enhance the performance of semiconductor devices, such as by enhancing the mobility of the charge carriers. For example, U.S. Patent Application No. 2003/0057416 to Currie et al. discloses strained material layers of silicon, silicon-germanium, and relaxed silicon and also including impurity-free zones that would otherwise cause performance degradation. The resulting biaxial strain in the upper silicon layer alters the carrier mobilities enabling higher speed and/or lower power devices. Published U.S. Patent Application No. 2003/0034529 to Fitzgerald et al. discloses a CMOS inverter also based upon similar strained silicon technology.

U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor device including a silicon and carbon layer sandwiched between silicon layers so that the conduction band and valence band of the second silicon layer receive a tensile strain. Electrons having a smaller effective mass, and which have been induced by an electric field applied to the gate electrode, are confined in the second silicon layer, thus, an re-channel MOSFET is asserted to have a higher mobility.

U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice in which a plurality of layers, less than eight monolayers, and containing a fractional or binary or a binary compound semiconductor layer, are alternately and epitaxially grown. The direction of main current flow is perpendicular to the layers of the superlattice.

U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si—Ge short period superlattice with higher mobility achieved by reducing alloy scattering in the superlattice. Along these lines, U.S. Pat. No. 5,683,934 to Candelaria discloses an enhanced mobility MOSFET including a channel layer comprising an alloy of silicon and a second material substitutionally present in the silicon lattice at a percentage that places the channel layer under tensile stress.

U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structure comprising two barrier regions and a thin epitaxially grown semiconductor layer sandwiched between the barriers. Each barrier region consists of alternate layers of SiO2/Si with a thickness generally in a range of two to six monolayers. A much thicker section of silicon is sandwiched between the barriers.

An article entitled “Phenomena in silicon nanostructure devices” also to Tsu and published online Sep. 6, 2000 by Applied Physics and Materials Science & Processing, pp. 391-402 discloses a semiconductor-atomic superlattice (SAS) of silicon and oxygen. The Si/O superlattice is disclosed as useful in a silicon quantum and light-emitting devices. In particular, a green electroluminescence diode structure was constructed and tested. Current flow in the diode structure is vertical, that is, perpendicular to the layers of the SAS. The disclosed SAS may include semiconductor layers separated by adsorbed species such as oxygen atoms, and CO molecules. The silicon growth beyond the adsorbed monolayer of oxygen is described as epitaxial with a fairly low defect density. One SAS structure included a 1.1 nm thick silicon portion that is about eight atomic layers of silicon, and another structure had twice this thickness of silicon. An article to Luo et al. entitled “Chemical Design of Direct-Gap Light-Emitting Silicon” published in Physical Review Letters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the light emitting SAS structures of Tsu.

U.S. Pat. No. 7,105,895 to Wang et al. discloses a barrier building block of thin silicon and oxygen, carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to thereby reduce current flowing vertically through the lattice more than four orders of magnitude. The insulating layer/barrier layer allows for low defect epitaxial silicon to be deposited next to the insulating layer.

Published Great Britain Patent Application 2,347,520 to Mears et al. discloses that principles of Aperiodic Photonic Band-Gap (APBG) structures may be adapted for electronic bandgap engineering. In particular, the application discloses that material parameters, for example, the location of band minima, effective mass, etc., can be tailored to yield new aperiodic materials with desirable band-structure characteristics. Other parameters, such as electrical conductivity, thermal conductivity and dielectric permittivity or magnetic permeability are disclosed as also possible to be designed into the material.

Furthermore, U.S. Pat. No. 6,376,337 to Wang et al. discloses a method for producing an insulating or barrier layer for semiconductor devices which includes depositing a layer of silicon and at least one additional element on the silicon substrate whereby the deposited layer is substantially free of defects such that epitaxial silicon substantially free of defects can be deposited on the deposited layer. Alternatively, a monolayer of one or more elements, preferably comprising oxygen, is absorbed on a silicon substrate. A plurality of insulating layers sandwiched between epitaxial silicon forms a barrier composite.

Despite the existence of such approaches, further enhancements may be desirable for using advanced semiconductor materials and processing techniques to achieve improved performance in semiconductor devices.

SUMMARY

A method for making a semiconductor device may include forming a first single crystal silicon layer having a first percentage of silicon 28, and forming a superlattice above the first single crystal silicon layer. The superlattice may include a plurality of stacked groups of layers, with each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base silicon portions. The method may further include forming a second single crystal silicon layer above the superlattice having a second percentage of silicon 28 higher than the first percentage of silicon 28.

By way of example, the first percentage of silicon 28 may be less than 93 percent, and the second percentage of silicon 28 may greater than 95 percent, and more particularly greater than 99 percent. In an example implementation, a third single crystal semiconductor layer may be formed between the first single crystal semiconductor layer and the superlattice and having a third percentage of silicon 28 higher than the first percentage of silicon 28. In still another example embodiment, a third single crystal semiconductor layer may be formed between the superlattice and the second single crystal semiconductor layer.

In one example implementation, the superlattice layer may comprise a first superlattice layer above the first single crystal semiconductor layer, and the method may further include forming a third single crystal semiconductor layer above the first superlattice, and forming a second superlattice above the third single crystal semiconductor layer and below the second single crystal semiconductor layer. In accordance with another example, the superlattice layer may be on the first single crystal silicon layer, and the second single crystal silicon layer may be on the superlattice layer.

The first single crystal silicon layer may have a first thickness and the second single crystal silicon layer may have a second thickness less than the first thickness, for example. In an example embodiment, the method may also include forming at least one circuit device associated with the second single crystal silicon layer. By way of example, the at least one circuit device may comprise a plurality of quantum bit devices. In accordance with another example, forming the at least one circuit device may include forming spaced apart source and drain regions in the second single crystal silicon layer defining a channel therebetween, and forming a gate comprising a gate dielectric layer overlying the channel and a gate electrode overlying the gate dielectric layer. By way of example, the at least one non-semiconductor monolayer may comprise oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged schematic cross-sectional view of a superlattice for use in a semiconductor device in accordance with an example embodiment.

FIG. 2 is a perspective schematic atomic diagram of a portion of the superlattice shown in FIG. 1.

FIG. 3 is a greatly enlarged schematic cross-sectional view of another embodiment of a superlattice in accordance with an example embodiment.

FIG. 4A is a graph of the calculated band structure from the gamma point (G) for both bulk silicon as in the prior art, and for the 4/1 Si/O superlattice as shown in FIGS. 1-2.

FIG. 4B is a graph of the calculated band structure from the Z point for both bulk silicon as in the prior art, and for the 4/1 Si/O superlattice as shown in FIGS. 1-2.

FIG. 4C is a graph of the calculated band structure from both the gamma and Z points for both bulk silicon as in the prior art, and for the 5/1/3/1 Si/O superlattice as shown in FIG. 3.

FIG. 5 is a schematic block diagram of a semiconductor device including an enriched ²⁸Si epitaxial layer formed on a superlattice in accordance with an example embodiment.

FIG. 6 is a schematic block diagram of a semiconductor device including a MOSFET formed on an enriched ²⁸Si epitaxial layer in accordance with an example embodiment.

FIG. 7 is a schematic block diagram of a semiconductor device including a quantum bit (qubit) device formed on an enriched ²⁸Si epitaxial layer in accordance with an example embodiment.

FIG. 8 is a schematic block diagram of an alternative embodiment of the semiconductor device of FIG. 5.

FIG. 9 is a schematic block diagram of another alternative embodiment of the semiconductor device of FIG. 5.

FIG. 10 is a schematic block diagram of an still another alternative embodiment of the semiconductor device of FIG. 5.

FIG. 11 is a flow diagram illustrating method aspects associated with the fabrication of the semiconductor devices of FIGS. 5-10.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which the example embodiments are shown. The embodiments may, however, be implemented in many different forms and should not be construed as limited to the specific examples set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in different embodiments.

Generally speaking, the present disclosure relates to the formation of semiconductor devices utilizing an enhanced semiconductor superlattice. The enhanced semiconductor superlattice is also referred to as an “MST” layer/film or “MST technology” in this disclosure.

More particularly, the MST technology relates to advanced semiconductor materials such as the superlattice 25 described further below. Applicant theorizes, without wishing to be bound thereto, that certain superlattices as described herein reduce the effective mass of charge carriers and that this thereby leads to higher charge carrier mobility. Effective mass is described with various definitions in the literature. As a measure of the improvement in effective mass Applicant's use a “conductivity reciprocal effective mass tensor”, M_(e) ⁻¹ and M_(h) ⁻¹ for electrons and holes respectively, defined as:

${M_{e,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{\sum\limits_{E > E_{F}}{\int\limits_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}d^{3}k}}}{\sum\limits_{E > E_{F}}{\int\limits_{B.Z.}{{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}d^{3}k}}}$

for electrons and:

${M_{h,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{- {\sum\limits_{E < E_{F}}{\int\limits_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}d^{3}k}}}}{\sum\limits_{E < E_{F}}{\int\limits_{B.Z.}{\left( {1 - {f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}} \right)d^{3}k}}}$

for holes, where f is the Fermi-Dirac distribution, E_(F) is the Fermi energy, T is the temperature, E(k,n) is the energy of an electron in the state corresponding to wave vector k and the n^(th) energy band, the indices i and j refer to Cartesian coordinates x, y and z, the integrals are taken over the Brillouin zone (B.Z.), and the summations are taken over bands with energies above and below the Fermi energy for electrons and holes respectively.

Applicant's definition of the conductivity reciprocal effective mass tensor is such that a tensorial component of the conductivity of the material is greater for greater values of the corresponding component of the conductivity reciprocal effective mass tensor. Again, Applicant theorizes without wishing to be bound thereto that the superlattices described herein set the values of the conductivity reciprocal effective mass tensor so as to enhance the conductive properties of the material, such as typically for a preferred direction of charge carrier transport. The inverse of the appropriate tensor element is referred to as the conductivity effective mass. In other words, to characterize semiconductor material structures, the conductivity effective mass for electrons/holes as described above and calculated in the direction of intended carrier transport is used to distinguish improved materials.

Applicant has identified improved materials or structures for use in semiconductor devices. More specifically, Applicant has identified materials or structures having energy band structures for which the appropriate conductivity effective masses for electrons and/or holes are substantially less than the corresponding values for silicon. In addition to the enhanced mobility characteristics of these structures, they may also be formed or used in such a manner that they provide piezoelectric, pyroelectric, and/or ferroelectric properties that are advantageous for use in a variety of different types of devices, as will be discussed further below.

Referring now to FIGS. 1 and 2, the materials or structures are in the form of a superlattice 25 whose structure is controlled at the atomic or molecular level and may be formed using known techniques of atomic or molecular layer deposition. The superlattice 25 includes a plurality of layer groups 45 a-45 n arranged in stacked relation, as perhaps best understood with specific reference to the schematic cross-sectional view of FIG. 1.

Each group of layers 45 a-45 n of the superlattice 25 illustratively includes a plurality of stacked base semiconductor monolayers 46 defining a respective base semiconductor portion 46 a-46 n and an energy band-modifying layer 50 thereon. The energy band-modifying layers 50 are indicated by stippling in FIG. 1 for clarity of illustration.

The energy band-modifying layer 50 illustratively includes one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. By “constrained within a crystal lattice of adjacent base semiconductor portions” it is meant that at least some semiconductor atoms from opposing base semiconductor portions 46 a-46 n are chemically bound together through the non-semiconductor monolayer 50 therebetween, as seen in FIG. 2. Generally speaking, this configuration is made possible by controlling the amount of non-semiconductor material that is deposited on semiconductor portions 46 a-46 n through atomic layer deposition techniques so that not all (i.e., less than full or 100% coverage) of the available semiconductor bonding sites are populated with bonds to non-semiconductor atoms, as will be discussed further below. Thus, as further monolayers 46 of semiconductor material are deposited on or over a non-semiconductor monolayer 50, the newly deposited semiconductor atoms will populate the remaining vacant bonding sites of the semiconductor atoms below the non-semiconductor monolayer.

In other embodiments, more than one such non-semiconductor monolayer may be possible. It should be noted that reference herein to a non-semiconductor or semiconductor monolayer means that the material used for the monolayer would be a non-semiconductor or semiconductor if formed in bulk. That is, a single monolayer of a material, such as silicon, may not necessarily exhibit the same properties that it would if formed in bulk or in a relatively thick layer, as will be appreciated by those skilled in the art.

Applicant theorizes without wishing to be bound thereto that energy band-modifying layers 50 and adjacent base semiconductor portions 46 a-46 n cause the superlattice 25 to have a lower appropriate conductivity effective mass for the charge carriers in the parallel layer direction than would otherwise be present. Considered another way, this parallel direction is orthogonal to the stacking direction. The band modifying layers 50 may also cause the superlattice 25 to have a common energy band structure, while also advantageously functioning as an insulator between layers or regions vertically above and below the superlattice.

Moreover, this superlattice structure may also advantageously act as a barrier to dopant and/or material diffusion between layers vertically above and below the superlattice 25. These properties may thus advantageously allow the superlattice 25 to provide an interface for high-K dielectrics which not only reduces diffusion of the high-K material into the channel region, but which may also advantageously reduce unwanted scattering effects and improve device mobility, as will be appreciated by those skilled in the art.

It is also theorized that semiconductor devices including the superlattice 25 may enjoy a higher charge carrier mobility based upon the lower conductivity effective mass than would otherwise be present. In some embodiments, and as a result of the band engineering achieved by the present invention, the superlattice 25 may further have a substantially direct energy bandgap that may be particularly advantageous for opto-electronic devices, for example.

The superlattice 25 also illustratively includes a cap layer 52 on an upper layer group 45 n. The cap layer 52 may comprise a plurality of base semiconductor monolayers 46. The cap layer 52 may have between 2 to 100 monolayers of the base semiconductor, and, more preferably between 10 to 50 monolayers.

Each base semiconductor portion 46 a-46 n may comprise a base semiconductor selected from the group consisting of Group IV semiconductors, Group III-V semiconductors, and Group II-VI semiconductors. Of course, the term Group IV semiconductors also includes Group IV-IV semiconductors, as will be appreciated by those skilled in the art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example.

Each energy band-modifying layer 50 may comprise a non-semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, carbon and carbon-oxygen, for example. The non-semiconductor is also desirably thermally stable through deposition of a next layer to thereby facilitate manufacturing. In other embodiments, the non-semiconductor may be another inorganic or organic element or compound that is compatible with the given semiconductor processing as will be appreciated by those skilled in the art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example

It should be noted that the term monolayer is meant to include a single atomic layer and also a single molecular layer. It is also noted that the energy band-modifying layer 50 provided by a single monolayer is also meant to include a monolayer wherein not all of the possible sites are occupied (i.e., there is less than full or 100% coverage). For example, with particular reference to the atomic diagram of FIG. 2, a 4/1 repeating structure is illustrated for silicon as the base semiconductor material, and oxygen as the energy band-modifying material. Only half of the possible sites for oxygen are occupied in the illustrated example.

In other embodiments and/or with different materials this one-half occupation would not necessarily be the case as will be appreciated by those skilled in the art. Indeed, it can be seen even in this schematic diagram, that individual atoms of oxygen in a given monolayer are not precisely aligned along a flat plane as will also be appreciated by those of skill in the art of atomic deposition. By way of example, a preferred occupation range is from about one-eighth to one-half of the possible oxygen sites being full, although other numbers may be used in certain embodiments.

Silicon and oxygen are currently widely used in conventional semiconductor processing, and, hence, manufacturers will be readily able to use these materials as described herein. Atomic or monolayer deposition is also now widely used. Accordingly, semiconductor devices incorporating the superlattice 25 in accordance with the invention may be readily adopted and implemented, as will be appreciated by those skilled in the art.

It is theorized without Applicant wishing to be bound thereto that for a superlattice, such as the Si/O superlattice, for example, that the number of silicon monolayers should desirably be seven or less so that the energy band of the superlattice is common or relatively uniform throughout to achieve the desired advantages. The 4/1 repeating structure shown in FIGS. 1 and 2, for Si/O has been modeled to indicate an enhanced mobility for electrons and holes in the X direction. For example, the calculated conductivity effective mass for electrons (isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice in the X direction it is 0.12 resulting in a ratio of 0.46. Similarly, the calculation for holes yields values of 0.36 for bulk silicon and 0.16 for the 4/1 Si/O superlattice resulting in a ratio of 0.44.

While such a directionally preferential feature may be desired in certain semiconductor devices, other devices may benefit from a more uniform increase in mobility in any direction parallel to the groups of layers. It may also be beneficial to have an increased mobility for both electrons and holes, or just one of these types of charge carriers as will be appreciated by those skilled in the art.

The lower conductivity effective mass for the 4/1 Si/O embodiment of the superlattice 25 may be less than two-thirds the conductivity effective mass than would otherwise occur, and this applies for both electrons and holes. Of course, the superlattice 25 may further comprise at least one type of conductivity dopant therein, as will also be appreciated by those skilled in the art.

Indeed, referring now additionally to FIG. 3, another embodiment of a superlattice 25′ in accordance with the invention having different properties is now described. In this embodiment, a repeating pattern of 3/1/5/1 is illustrated. More particularly, the lowest base semiconductor portion 46 a′ has three monolayers, and the second lowest base semiconductor portion 46 b′ has five monolayers. This pattern repeats throughout the superlattice 25′. The energy band-modifying layers 50′ may each include a single monolayer. For such a superlattice 25′ including Si/O, the enhancement of charge carrier mobility is independent of orientation in the plane of the layers. Those other elements of FIG. 3 not specifically mentioned are similar to those discussed above with reference to FIG. 1 and need no further discussion herein.

In some device embodiments, all of the base semiconductor portions of a superlattice may be a same number of monolayers thick. In other embodiments, at least some of the base semiconductor portions may be a different number of monolayers thick. In still other embodiments, all of the base semiconductor portions may be a different number of monolayers thick.

In FIGS. 4A-4C, band structures calculated using Density Functional Theory (DFT) are presented. It is well known in the art that DFT underestimates the absolute value of the bandgap. Hence all bands above the gap may be shifted by an appropriate “scissors correction.” However, the shape of the band is known to be much more reliable. The vertical energy axes should be interpreted in this light.

FIG. 4A shows the calculated band structure from the gamma point (G) for both bulk silicon (represented by continuous lines) and for the 4/1 Si/O superlattice 25 shown in FIG. 1 (represented by dotted lines). The directions refer to the unit cell of the 4/1 Si/O structure and not to the conventional unit cell of Si, although the (001) direction in the figure does correspond to the (001) direction of the conventional unit cell of Si, and, hence, shows the expected location of the Si conduction band minimum. The (100) and (010) directions in the figure correspond to the (110) and (−110) directions of the conventional Si unit cell. Those skilled in the art will appreciate that the bands of Si on the figure are folded to represent them on the appropriate reciprocal lattice directions for the 4/1 Si/O structure.

It can be seen that the conduction band minimum for the 4/1 Si/O structure is located at the gamma point in contrast to bulk silicon (Si), whereas the valence band minimum occurs at the edge of the Brillouin zone in the (001) direction which we refer to as the Z point. One may also note the greater curvature of the conduction band minimum for the 4/1 Si/O structure compared to the curvature of the conduction band minimum for Si owing to the band splitting due to the perturbation introduced by the additional oxygen layer.

FIG. 4B shows the calculated band structure from the Z point for both bulk silicon (continuous lines) and for the 4/1 Si/O superlattice 25 (dotted lines). This figure illustrates the enhanced curvature of the valence band in the (100) direction.

FIG. 4C shows the calculated band structure from both the gamma and Z point for both bulk silicon (continuous lines) and for the 5/1/3/1 Si/O structure of the superlattice 25′ of FIG. 3 (dotted lines). Due to the symmetry of the 5/1/3/1 Si/O structure, the calculated band structures in the (100) and (010) directions are equivalent. Thus, the conductivity effective mass and mobility are expected to be isotropic in the plane parallel to the layers, i.e. perpendicular to the (001) stacking direction. Note that in the 5/1/3/1 Si/O example the conduction band minimum and the valence band maximum are both at or close to the Z point.

Although increased curvature is an indication of reduced effective mass, the appropriate comparison and discrimination may be made via the conductivity reciprocal effective mass tensor calculation. This leads Applicant to further theorize that the 5/1/3/1 superlattice 25′ should be substantially direct bandgap. As will be understood by those skilled in the art, the appropriate matrix element for optical transition is another indicator of the distinction between direct and indirect bandgap behavior.

An example approach for fabricating a semiconductor device 150 using the above-described superlattice structures to provide an enriched ²⁸Si active device layer is first described with reference to FIG. 5 and the flow diagram 80 of FIG. 11. By way of background, silicon has multiple natural stable isotopes. The most abundant natural stable isotopes are ²⁸Si (92.23%), ²⁰Si (4.67%), and ³⁰Si (3.10%). There are several advantages to ²⁸Si substrates. For example, they have higher thermo-conductivity (better heat dissipation), and a higher decoherence time which is useful for qubit applications.

On the other hand, there is a substantial cost related to the purification of ²⁸Si, and thus production of ²⁸Si in large quantities (e.g., as a substrate) can be cost prohibitive. As a result, some attempts have been made to form ²⁸Si layers on top of a natural silicon substrate (i.e., having 92.23% or less ²⁸Si).However, due to silicon interdiffusion, a relatively thick ²⁸Si epitaxial layer still needs to be grown on the substrate. In still another approach, to prevent silicon intermixing, designs utilizing a silicon-on-insulator (SOI) approach have also been proposed. While this allows for a relatively thin ²⁸Si layer, the SOI technology used for this implementation is costly as well.

In the illustrated example, beginning at Block 81, a first single crystal silicon layer 151 (e.g., a substrate) is provided having a first percentage of ²⁸Si, at Block 82. Furthermore, a superlattice 125 is grown on the first single crystal layer 151 (Block 83), such as the Si/O superlattice structures described further above. Additionally, a second single crystal silicon layer 152 (e.g., an active device layer) is epitaxially grown on the superlattice 125, at Block 84. More particularly, the second percentage of ²⁸Si is higher than the first percentage of ²⁸Si, defining an isotropically enriched, high concentration ²⁸Si layer. The method of FIG. 8 illustratively concludes at Block 85.

The first silicon layer 151 has a first thickness, and the second silicon layer 152 has a second thickness less than the first thickness. In other words, the first silicon layer 151 may serve as the substrate for the semiconductor device 150, while the second silicon layer 152 may serve as an epitaxial active layer in which additional circuitry may be formed to take advantage of the enhanced ²⁸Si properties, yet at relatively low fabrication costs. In the illustrated configuration, the superlattice 125 advantageously acts as a physical barrier to help prevent intermixing of the first layer 151 with ²⁸Si<93% and the second layer 152 with ²⁸Si>95%.

Referring additionally to FIG. 6, in accordance with one example implementation of a semiconductor device 150′, the additional circuitry illustratively includes one or more MOSFET devices (e.g., CMOS) associated with the second silicon layer 152′. More particularly, the MOSFET illustratively includes spaced apart source and drain regions 153′, 154′ in the second single crystal silicon layer 152′ defining a channel 159′ therebetween, and a gate 155′ including a gate dielectric layer 156′ (e.g., SiO₂) overlying the channel and a gate electrode 157′ overlying the gate dielectric layer. Sidewall spacers 158′ are also formed adjacent the gate 155′. In this example, the first silicon layer 151′ has less than 93% ²⁸Si, while the second silicon layer 152′ has at least 95% ²⁸Si, although different percentages may be used in different embodiments.

Turning to FIG. 7, in accordance with another example implementation a semiconductor device 150″ illustratively includes one or more quantum bit (qubit) devices 160″ associated with the second silicon layer 152″. More particularly, the quantum bit device 160″ illustratively includes an insulating layer 161″ (e.g., SiO₂) on the second silicon layer 152″, and a gate electrode 160″ on the insulating layer defining a hole or electron isolation area 163″ beneath the gate electrode in the second single crystal silicon layer. In this example, the first silicon layer 151″ has less than 93% ²⁸Si, while the second silicon layer 152″ has at least 99% ²⁸Si, although different percentages may be used in different embodiments. Further implementations details and examples of quantum devices which may be used are set forth in the following references, which are hereby incorporated herein in their entireties by reference: U.S. Pat. No. 9,886,668 to Dzurak et al; “Coherent spin control of s-, p-, d- and f-electrons in a silicon quantum dot” by Leon et al. (Nature Communications, (2020) 11:797); “Single-spin qubits in isotropically enriched silicon at low magnetic field” by Zhao et al. (Nature Communications, (2019) 10:5500); and “Silicon CMOS architecture for a spin-based quantum computer” by Veldhorst et al. (Nature Communications, (2017) 8:1766).

Turning now to FIG. 8, another example embodiment of a semiconductor device 250 illustratively includes a first single crystal silicon layer 251 (e.g., a substrate) having a first percentage of ²⁸Si, a superlattice 225, and a second single crystal silicon layer 252 (e.g., an active device layer) similar to those discussed above with respect to FIG. 5. However, in the present example a third single crystal semiconductor layer 253 is epitaxially grown on the first layer 253, and the superlattice 225 is formed on the third single crystal semiconductor layer. More particularly, the third single crystal semiconductor layer 253 has a third percentage of ²⁸Si which is also higher than the first percentage of ²⁸Si, defining an isotropically enriched, high concentration ²⁸Si layer. For example, the third single crystal semiconductor layer 253 may be used as a seed layer to start the transition from the lower (first) percentage of ²⁸Si to the higher (second) percentage ²⁸Si before depositing the superlattice layer 225. In an example embodiment, the concentration of ²⁸Si may be graded or increase from the bottom of the layer to the top, or the concentration of ²⁸Si may be relatively consistent across the third layer in some embodiments.

The silicon monolayers 46 of the superlattice 225 may also be formed with enriched ²⁸Si. In this regard, it should be noted that in some embodiments, the third layer 253 may be absent, but the transition to the enriched ²⁸Si may take place in the silicon monolayers 46 of the superlattice 225. That is, some or all of the monolayers 46 of the superlattice 225 may be formed with enriched ²⁸Si, with or without the third layer 225.

Turning now to FIG. 9, in another example embodiment a semiconductor device 350 illustratively includes a first single crystal silicon layer 351 (e.g., a substrate) having a first percentage of ²⁸Si, a superlattice 325, and a second single crystal silicon layer 352 (e.g., an active device layer) similar to those discussed above with respect to FIG. 5. However, in the present example a third single crystal semiconductor layer 353 is epitaxially grown on the superlattice 325, and is accordingly between the superlattice and the second single crystal semiconductor layer 352.

In this configuration, rather than providing a physical barrier to intermixing of silicon from the first and second layers 351, 352 as with the device 250, here the interstitial trapping properties of the superlattice 325 help prevent intermixing by eliminating silicon interstitials from the system. Interstitials contribute to silicon self-diffusion, which leads to in silicon intermixing. To target this, the depth of the superlattice 325 may be set by the thickness of the third layer 353 to the desired distance under the transition region or interface 354. Further details regarding use of superlattices to help reduce silicon interstitials are provided in U.S. Pat. No. 10,580,866 to Takeuchi et al. and U.S. Pat. No. 9,941,359 to Mears et al., which are both hereby incorporated herein in their entireties by reference. The third layer 353 may have the same or similar ²⁸Si concentration to that of the first layer 351, for example, making the superlattice 325 a “buried” layer with respect to the transition region 354. However, in some embodiments the third layer 353 may also have an enhanced ²⁸Si concentration as discussed above with reference to FIG. 8.

Referring additionally to FIG. 10, an example semiconductor device 450 in which multiple superlattices 425 a, 425 b are utilized for both interstitial trapping and as a physical barrier is now described. More particularly, the semiconductor device 450 illustratively includes a first single crystal silicon layer 451 (e.g., a substrate) having a first percentage of ²⁸Si, a first superlattice 425 a on the first single crystal silicon layer, a third silicon layer 453 on the first superlattice, the second superlattice 425 b on the third silicon layer, and a second single crystal silicon layer 452 (e.g., an active device layer). The first, second, and third layers 451-453 may be similar to layers 351-353 discussed above with respect to FIG. 9. Accordingly, this configuration advantageously provides a combination of interstitial trapping and a physical barrier to help prevent the intermixing of silicon atoms between the first and second layers 451, 452.

The foregoing embodiments provide a relatively low cost approach for growing purified ²⁸Si layers on a silicon substrate using the above-described superlattice structures. In addition to the above-noted advantages of ²⁸Si, the above-described configurations provide additional advantages as a result of the incorporated superlattice(s). More particularly, in addition to the relatively low cost fabrication as a result of the superlattice(s), the superlattice(s) advantageously help prevent silicon intermixing, allowing for a relatively thin ²⁸Si epitaxial (active) layer. Additionally, as noted above, the superlattice(s) can help reduce silicon interstitials from the 28Si epitaxial layer, as discussed further in the aforementioned '866 and '359 patents. This helps to even further decrease interdiffusion. Additionally, elimination of interstitial point defects increases the effective silicon purity, allowing for even higher quantum decoherence times for quantum device applications.

Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims. 

That which is claimed is:
 1. A method for making a semiconductor device comprising: forming a first single crystal silicon layer having a first percentage of silicon 28; forming a superlattice above the first single crystal silicon layer, the superlattice comprising a plurality of stacked groups of layers, each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base silicon portions; and forming a second single crystal silicon layer above the superlattice having a second percentage of silicon 28 higher than the first percentage of silicon
 28. 2. The method of claim 1 wherein the first percentage of silicon 28 is less than 93 percent.
 3. The method of claim 1 wherein the second percentage of silicon 28 is greater than 95 percent.
 4. The method of claim 1 wherein the second percentage of silicon 28 is greater than 99 percent.
 5. The method of claim 1 further comprising forming a third single crystal semiconductor layer between the first single crystal semiconductor layer and the superlattice and having a third percentage of silicon 28 higher than the first percentage of silicon
 28. 6. The method of claim 1 further comprising forming a third single crystal semiconductor layer between the superlattice and the second single crystal semiconductor layer.
 7. The method of claim 1 wherein the superlattice layer comprises a first superlattice layer above the first single crystal semiconductor layer; and further comprising: forming a third single crystal semiconductor layer above the first superlattice; and forming a second superlattice above the third single crystal semiconductor layer and below the second single crystal semiconductor layer.
 8. The method of claim 1 wherein the superlattice layer is on the first single crystal silicon layer, and the second single crystal silicon layer is on the superlattice layer.
 9. The method of claim 1 wherein the first single crystal silicon layer has a first thickness and the second single crystal silicon layer has a second thickness less than the first thickness.
 10. The method of claim 1 further comprising forming at least one circuit device associated with the second single crystal silicon layer.
 11. The method of claim 10 wherein the at least one circuit device comprises a plurality of quantum bit devices.
 12. The method of claim 10 wherein forming the at least one circuit device comprises: forming spaced apart source and drain regions in the second single crystal silicon layer defining a channel therebetween; and forming a gate comprising a gate dielectric layer overlying the channel and a gate electrode overlying the gate dielectric layer.
 13. The method of claim 1 wherein the at least one non-semiconductor monolayer comprises oxygen.
 14. A method for making a semiconductor device comprising: forming a first single crystal silicon layer having a first percentage of silicon 28 less than 93 percent; forming a superlattice above the first single crystal silicon layer, the superlattice comprising a plurality of stacked groups of layers, each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least oxygen monolayer constrained within a crystal lattice of adjacent base silicon portions; and forming a second single crystal silicon layer above the superlattice having a second percentage of silicon 28 higher than 95 percent.
 15. The method of claim 14 wherein the second percentage of silicon 28 is greater than 99 percent.
 16. The method of claim 14 further comprising forming a third single crystal semiconductor layer between the first single crystal semiconductor layer and the superlattice and having a third percentage of silicon 28 higher than the first percentage of silicon
 28. 17. The method of claim 14 further comprising forming a third single crystal semiconductor layer between the superlattice and the second single crystal semiconductor layer.
 18. The method of claim 14 wherein the superlattice layer comprises a first superlattice layer above the first single crystal semiconductor layer; and further comprising: forming a third single crystal semiconductor layer above the first superlattice; and forming a second superlattice above the third single crystal semiconductor layer and below the second single crystal semiconductor layer.
 19. The method of claim 14 wherein the superlattice layer is on the first single crystal silicon layer, and the second single crystal silicon layer is on the superlattice layer.
 20. A method for making a semiconductor device comprising: forming a first single crystal silicon layer having a first percentage of silicon 28; forming a superlattice above the first single crystal silicon layer, the superlattice comprising a plurality of stacked groups of layers, each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least one oxygen monolayer constrained within a crystal lattice of adjacent base silicon portions; forming a second single crystal silicon layer above the superlattice having a second percentage of silicon 28 higher than the first percentage of silicon 28; and forming at least one circuit device associated with the second single crystal silicon layer.
 21. The method of claim 20 wherein the first percentage of silicon 28 is less than 93 percent, and the second percentage of silicon 28 is greater than 95 percent.
 22. The method of claim 20 further comprising forming a third single crystal semiconductor layer between the first single crystal semiconductor layer and the superlattice and having a third percentage of silicon 28 higher than the first percentage of silicon
 28. 23. The method of claim 20 further comprising forming a third single crystal semiconductor layer between the superlattice and the second single crystal semiconductor layer.
 24. The method of claim 20 wherein the superlattice layer comprises a first superlattice layer above the first single crystal semiconductor layer; and further comprising: forming a third single crystal semiconductor layer above the first superlattice; and forming a second superlattice above the third single crystal semiconductor layer and below the second single crystal semiconductor layer.
 25. The method of claim 20 wherein the superlattice layer is on the first single crystal silicon layer, and the second single crystal silicon layer is on the superlattice layer.
 26. The method of claim 20 wherein the at least one circuit device comprises a plurality of quantum bit devices.
 27. The method of claim 20 wherein forming the at least one circuit device comprises: forming spaced apart source and drain regions in the second single crystal silicon layer defining a channel therebetween; and forming a gate comprising a gate dielectric layer overlying the channel and a gate electrode overlying the gate dielectric layer. 